Wheel torque disturbance suppression control for vehicle hold before/during engine startup

ABSTRACT

A vehicle-hold control method is used to hold an engine-powered vehicle from rolling backward on a road gradient during an engine start. The engine shuts down when the vehicle is stopped following operation of the vehicle under engine power. The engine is re-started during an engine cranking interval as vehicle wheel brakes are applied. The brakes are released in response to detection of pre-calibrated engine speed characteristics.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/949,068 filed Nov. 18, 2010, now U.S. Pat. No. 8,579,765, which, inturn, claims the benefit of U.S. provisional application Ser. No.61/267,983 filed Dec. 9, 2009, the disclosures of which are herebyincorporated in their entirety by reference herein.

BACKGROUND

During travel of an engine-powered vehicle, there are many instanceswhen the vehicle must stop before a destination is reached. This mayoccur, for example, when the vehicle stops at traffic signals,cross-walks, stop signs and the like. A so-called micro-hybrid vehiclemay enable a stop/start strategy for starting and stopping the vehicleengine during a driving event. The engine is shut down if no power isrequired (e.g. while waiting at a traffic light). As soon as power isrequested, the engine is automatically restarted. By avoidingunnecessary engine idling, the vehicle's fuel economy will be improved.For this reason, it is desirable to use the engine shut down function asmuch as possible when certain engine stop conditions are satisfied.

For a conventional vehicle launch on a hill, a hill-start brake holdfunction, using a driver-activated brake, may be used to help the driverlaunch the vehicle on an uphill gradient by preventing the vehicle fromrolling backward. The brake hold function is usually activated when theestimated road gradient is higher than a calibrated threshold level; forexample about 7% for vehicles with automatic torque convertertransmissions. Below this threshold, powertrain creep torque issufficient to counteract a negative road gradient load on the vehiclesuch that the vehicle begins to move forward once the driver actuatedbrake is released.

In the following description, a 7% gradient threshold will be used indescribing an example embodiment without loss of generality.

Vehicle torque at zero accelerator pedal input is:T _(Veh) =T _(PT) +T _(Brk) −T _(Rl),where T_(Brk) is brake torque, T_(PT) is the powertrain torque at wheelsand T_(PT)=T_(Creep) when the vehicle is idling. T_(Creep) is creeptorque and T_(Rl) is road load due to gravity.

Brake torque at vehicle standstill is:T _(Brk) =T _(Rl) −T _(Creep) (assuming sufficient brake pressure input)

Gross torque on the vehicle at vehicle standstill is:T _(Veh)=0

Gross torque on vehicle during vehicle launch after brake release is:T _(Veh) =T _(PT) −T _(Rl)and before pressing accelerator pedal:T _(Veh) =T _(Creep) −T _(Rl)T _(Creep) −T _(Rl)>0 when the road gradient is less than say 7%.T _(Creep) −T _(Rl)<0 when the road gradient is larger than 7%.

In contrast to a conventional vehicle, a micro-hybrid electric vehicleenables an engine stop-start function at vehicle standstill. It ispossible for the vehicle to roll backward downhill after brake releaseand before the engine is started, where T_(PT)=0, even if the gradientis less than 7% (it is assumed here that the brake hold function willtake care of the hill start assistance for gradients higher than 7%).The backward roll is due to the absence of powertrain creep torqueduring engine stop.

Gross torque on vehicle during launch after brake release, but beforeengine start, is:T _(Veh) =−T _(Rl)<0where T_(Veh) is gross torque on the vehicle due to gravity and −T_(Rl)is a negative torque component in a direction parallel to the roadway.

Such a negative torque on the vehicle may cause the vehicle to roll backand even stall the engine during its startup process. The effect of thenegative torque also depends on the vehicle load condition. This isdemonstrated by the following relationship:T _(Rl)=(M _(vehicle) +M _(payload))g sin (θ),where θ=road slope angle corresponding to the gradient, M_(vehicle)=massof the vehicle, and M_(payload)=vehicle payload mass.

The heavier the vehicle is loaded (M_(payload)), the higher the negativetorque on the vehicle.

It is desirable to hold a micro-hybrid vehicle at a standstill beforethe engine startup regardless of external conditions. One solution tothis problem is to reduce the brake hold gradient threshold down to alower gradient level, e.g. 3%, such that the vehicle launch process canbe supported by the brake hold function to avoid possible vehiclerollback when there is a road gradient. However, the following problemsmake the use of the brake hold function unfeasible for solving themicro-hybrid vehicle launch problem in a low road gradient range for thefollowing reasons.

1. The brake hold function depends on powertrain torque estimation todetermine when to release brake pressure. When the estimated powertraintorque {tilde over (T)}_(PT) is higher than the estimated gradient load{tilde over (T)}_(Rl), (i.e. T_(Veh)=T_(PT)−T_(Rl)>0), the brake holdwill start to release brake pressure. If it is not higher, it canmaintain the brake pressure close to the initial driver input level suchthat:T _(Veh) =T _(PT) +T _(Brk) −T _(Rl)=0, andT _(Brk) =T _(Rl) −T _(PT)>0.

The problem of compensating for the negative road gradient load beforethe engine startup then would be solved and there will be no vehiclerollback. But, this solution will present another problem. Thepowertrain torque estimation is far from accurate during an enginestartup process. It is only after the engine comes to a steady statethat the powertrain torque estimation will converge to the truepowertrain torque value. Consequently, if the estimated powertraintorque is too high, early brake pressure release will be commanded bythe brake hold function, which may result in unexpected vehicle rollbackmotion. On the other hand, if the estimated powertrain torque is toosmall, the brake pressure release will be delayed. This delay will dragthe vehicle launch attempt as demonstrated by the following equations:T _(Veh) =T _(PT) +T _(Brk) −T _(Rl)=0T _(Brk) =T _(Rl) −T _(PT)<0.Such a consequence is undesirable.

2. If the vehicle is standing on a near flat surface (e.g., −3%˜3%), thedriver does not expect any delay in vehicle launch. However, the roadgradient estimation usually does not have good accuracy, especially in alow gradient range. The road gradient estimation accuracy is alsoaffected by environmental conditions (e.g. temperature) and gradientsensor signal quality. As a result, lowering a brake hold gradientthreshold may bring a brake hold function induced vehicle launch delayin a flat ground vehicle launch, which sacrifices vehicle performance.

When the vehicle launch is in a low gradient range (e.g., −3%˜7%), thepowertrain creep torque is sufficient to overcome the road load torqueon the vehicle as the launch of the vehicle is started. In this range ofoperation, the brake hold function for vehicle launch assistance isexpected to be terminated once the vehicle creep torque will beavailable from the powertrain. The creep torque will move the vehicleforward without advancing the accelerator pedal. The presence of thecreep torque, even if there could be a small level of vehicle rollback,which happens in a case of an inaccurate gradient estimation, may causeslight vehicle motion. But that can be sufficiently and promptlycontrolled by most drivers. Driver intuition will help the driversuccessfully handle this situation. The brake hold function for hilllaunch assistance/hill hold control thus would give control authorityback to the driver by releasing the brake pressure control back to thedriver's input level as soon as powertrain torque will be available andsufficient for vehicle launch purposes.

SUMMARY

The present invention comprises a brake-based vehicle hill-startassistance function that will solve the problem of road grade levelvehicle hill launch assistance. The control objective is to keep thevehicle at standstill before engine startup and to enable prompt vehiclelaunch performance once the powertrain is ready to deliver drivingtorque to the vehicle traction wheels.

The brake-based control can be applied when the estimated road gradientis in a low level range. Further, a brake pressure release decision ismade based on powertrain state and engine speed rather than estimatedpowertrain torque output.

The brake-based control, which can be identified by the name WheelTorque Disturbance Suppression (WTDS), is an open loop brake controlfunction that can be used for initial combustion torque mitigation,which avoids powertrain torque oscillations during engine startup bykeeping the brakes applied. The control uses WTDS control logic for lowroad gradient vehicle hold purposes for hybrid vehicles that use anengine stop-start strategy. Reference may be made to pending U.S.Published patent Application No. 2010/0305790 A1 published Dec. 10,2010, filed May 28, 2009, and titled “Wheel Torque DisturbanceSuppression” for a description of WTDS control. That application isassigned to the assignee of the present invention.

The present invention further comprises a method for preventing amicro-hybrid vehicle from motion before engine startup using automaticengine stop-start strategy during a driving event. The method includesusing a driver activated service brake that builds up and/or maintainsbrake pressure during an engine-off state when the vehicle is stopped.Further, the method solves a powertrain torque spike problem observed ina micro-hybrid vehicle system during engine startup by applying brakebased technology.

Engine torque transients during engine cranking cause engine speedirregularities. The method of the invention may use peak engine speedwhen these irregularities occur. A detection of the peak engine speedmay be used to trigger a brake release. Detection of an instant whenengine speed crosses a pre-calibrated threshold also can be used totrigger a brake release during engine cranking. Further, a change inengine speed at a pre-calibrated rate during engine cranking could beused to trigger a brake release as the vehicle powertrain transitionsfrom an engine cranking mode directly to a launch mode. Still further,detection of an engine speed that is above a pre-calibrated thresholdfor a pre-calibrated time can be used to trigger a brake release.

The method of the invention avoids a late brake release, which coulddrag the vehicle launch. It also avoids an early brake release, whichcould cause a perceptible torque “bump.”

Although the embodiment of the invention herein described usespressure-operated wheel brakes to obtain friction brake torque, theinvention could instead use electric brakes to develop brake torque atthe wheels. The control variable used to initiate a hill hold functionthen could be brake actuator current, for example, rather than brakepressure.

If the roadway gradient is greater than the pre-determined value,estimated powertrain torque, as well as engine speed, could be useful todetermine a time to release the brake during cranking. When the gradientis greater than 7%, for example, the hill hold feature will stay appliedwhen the driver releases the brake pedal until powertrain torque isgreater than gradient torque, or until the accelerator pedal is advancedand the engine speed is higher than a threshold value. This is becausethe more the road gradient, the more the delay will be in a vehiclelaunch. Thus there will be more time for engine torque irregularities toconverge to an accurate torque value at the end of the cranking phase.This allows for more time for an accurate estimated powertrain torque todevelop.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a-1i are time plots of engine characteristics, vehicle speed,acceleration and brake pressures during a vehicle hold event;

FIG. 2 is a logic flow chart for a vehicle hold assistance feature; and

FIG. 3 is a schematic representation of a micro-hybrid electric vehiclepowertrain capable of embodying the invention.

DETAILED DESCRIPTION

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention that may be embodied in variousand alternative forms. The figures are not necessarily to scale; somefeatures may be exaggerated or minimized to show details of particularcomponents. Therefore, specific structural and functional detailsdisclosed herein are not to be interpreted as limiting, but merely as arepresentative basis for teaching one skilled in the art to variouslyemploy the present invention.

The WTDS brake-based control supports the stop and start performanceassociated with a micro-hybrid vehicle by holding the vehicle on agradient and preventing undesired motion using a hydraulic brake system(foundation brake system). It is an open loop system that provides anability to hold the brake pressure to a level that is either calibratedor determined adaptively to be sufficient for an engine startup torquespike suppression. This is a passive control and the assumption behindit is that the original driver brake pressure input is higher than thebrake-based hold pressure. For this reason, the engine shutdown entrycondition has to assume the same pressure threshold. That is, the engineshutdown will not be commanded if the driver does not apply sufficientbrake pedal input such that the brake master cylinder pressure is higherthan the brake-based hold pressure.

Given sufficient high brake line pressure, the brake reaction torquewill be able to hold the vehicle wheels from any motion caused bydisturbance torque at the wheels. The brake action actually isolates theundesirable powertrain disturbance torque from the vehicle. The brakepressure may be released, according to one feature of the invention,when an engine speed peak is detected and passed. A brake release may bereleased also when engine speed increases in the engine cranking mode ata calibrated rate or when engine speed is greater than a calibratedthreshold. As the brake pressure ramps down and the powertrain torqueincreases, vehicle creep or drive-off (launch) will take place inresponse to a driver's accelerator pedal command. The overall engineautomatic startup process during vehicle start will be smooth andcomfortable compared to a conventional vehicle (no stop/start function).The brake pressure hold function will be terminated immediately in thepresence of accelerator pedal apply or brake pedal-reapply.

The brake pressure vehicle-hold function is expected to hold the vehiclefrom motion before the engine startup but after the driver releases thebrake pedal. It also enables vehicle creep motion as soon as the enginebegins its start cycle so that it is ready to supply driving torque. Forroad gradients higher than 7%, it is assumed that the normal vehiclehold function will assist the vehicle launch process. The WTDS functionwill only be activated/enabled when there is an engine stop in thetransmission Drive range and the service brake system is unavailable tosupport the brake hold function.

FIG. 3 is a schematic drawing of a micro-hybrid vehicle system 10 thatmay incorporate the invention. It includes an engine 12 that may bedrivably coupled during engine starting to an enhanced engine startermotor 14, which may be selectably clutched to the engine crankshaft 17.Torque from engine 12 is delivered through transmission 16 andtransmission torque output shaft 22 to a torque transfer unit andvehicle traction wheels 30 and 31. Differential 23 divides engine torquedelivery to axle half shafts 28 and 29. A battery 13 powers the startermotor and an electrically driven pump 24, which supplies wheel brakepressure for a brake hold function. Transmission ratio ranges aremanually selected by shift lever system 44 with automatic ratio rangefeatures at 46 and a ratio range selector at 44. Ratio ranges aredelivered to module 42.

An engine control module 50 controls engine 12 and motor 14. Brake pedal54 and a brake activation system 51 provide driver brake signals to anautomatic brake control module 53 for wheel brakes 78 and 79. Brakepressure flow paths from pump 24 and the brake pressure control module53 to the wheel brakes are shown at 77 and 76. A vehicle systemcontroller 43 (VSC) provides supervisory control of modules 42, 50 and53 using a control area network CAN. Brake activation system 51 providesa brake state signal (BS) to the VSC and module 53 provides a brakepressure signal (BP) to the VSC. It also controls pressure delivery ofbrake cylinder pressure from pump 24 and brake cylinder pressuredeveloped by driver input at 54.

A gradient sensor 47 provides gradient data to vehicle system control43. Sensor 47 may determine a road gradient using, for example, anaccelerometer that provides a readout based in part on a gravity forcecomponent, or on data using an inclinometer.

The system of the invention holds the vehicle wheels and prevents thevehicle from moving before the engine startup using its ability to holdthe brake pressure once driver brake release is detected. The holdpressure level is expected to be as close to the original driver brakeinput before release as possible. It also has to be higher than a levelof master brake cylinder pressure that is either calibrated oradaptively determined for other brake based control purposes, e.g.initial combustion torque mitigation. This is depicted as a Phase Asection in FIGS. 1a -1 i.

The system can be designed with a passive brake pressure hold function.The assumption behind this is that the original driver brake pressureinput is sufficient to hold the vehicle in standstill in an absence ofpowertrain creep torque. It can also involve an active brake pressurebuild function in the case in which the brake pressure hold level isinsufficient to achieve vehicle hold at standstill. Given sufficienthigh brake line pressure, the brake static reactive friction torque willbe able to isolate the vehicle from the road gradient load and otherdisturbance torque on the vehicle. As a result, the vehicle is kept instandstill before the engine is being started.

The brake pressure release will be determined based on certain enginestartup powertrain timing. The brake release is requested; i.e. thesystem is set to inactive, when a certain powertrain state condition issatisfied. The engine speed signal is used as the powertrain stateindicator. The engine speed signal is both low pass filtered and highpass filtered to get a relatively smooth engine speed signal and itsderivative. The brake release is activated when an engine start flag isseen, which is determined by either condition 1 or condition 2 listedbelow:

Condition 1: A certain engine speed threshold has passed and enginestartup speed peak has been detected.

Condition 2: A certain high engine speed threshold has been passed for acertain calibration time duration as well as a certain level ofcalibrated engine speed gradient.

Besides normal brake pressure release conditions, the followingconditions are also used for making brake pressure release decision:

Condition 3: Driver brake re-apply detected.

Condition 4: Accelerator pedal motion detected.

Condition 5: WTDS timer expired.

If any of these brake pressure release conditions is satisfied, brakepressure will be released immediately to zero or to a current driverbrake input pressure level to give the control authority back to thedriver. The system only functions during engine startup up to apredetermined time period (Condition 5). In the case in which the enginedoes not startup correctly, a timer setting will be extended to keep itsresponsibility if a failed engine startup is followed immediately by anautomatic engine restart attempt. After a certain predetermined numberof attempts to restart, the system will be disabled if the engine doesnot work properly.

After brake pressure release is commanded, vehicle creep or drive-off(launch) will be exhibited according to a driver's accelerator pedalcommand as the brake pressure ramps down and the powertrain torqueincreases. The overall engine automatic startup process during vehiclestart will be smooth and comfortable compared to a conventional vehicle(no stop/start function). During a starting maneuver, the brake forcecan neither be reduced too early, whereby possible road gradient loadmay roll the vehicle backward, nor too late, whereby loss of drivabilityand degraded performance will be the result as the brake force drags thevehicle from driving-off. The WTDS use case for vehicle hold isillustrated in FIGS. 1a-1i . The control logic for vehicle hold is shownin FIG. 2.

In an alternative vehicle hold control system to keep the vehicle instandstill before the engine startup in drive, transmission tie-uptechnology can be applied to achieve the same function to assist vehiclehold given that the transmission tie-up release will be based on theCondition 1 and Condition 2 stated before.

The brake based vehicle hold control strategy has the followingadvantages over the alternative system:

1. Brake system has high friction force capacity and longer durabilityof friction materials. It is possible for a transmission tie-upmechanism to be damaged in a heavy vehicle loading condition. Thereplacement and cost to change a faded brake friction pad is much lesscostly than the cost of transmission control components.

2. The brake based method basically satisfies the control requirement bytaking advantage of the initial driver brake input. It is not necessaryto actively build pressure to that pressure required in a transmissioncontrol device.

3. The proposed brake based method consumes very little energy forelectromagnetic brake valves during its passive brake pressure holdfunction in comparison to the energy consumption for pump operation in atransmission based solution.

FIGS. 1a through 1h show time plots for a wheel torque disturbancesuppression function for a micro-hybrid electric vehicle hold functionduring an engine startup when the vehicle is on a road gradient of about−3% to approximately 7%. The time plots are made with time intervals A,B, and C.

As shown by plot 60 the brake pedal is released at 62 and the brakepedal pressure decreases to a near 0 value at 64 after the engine startfunction is ended. The engine start is triggered at time 62 andcontinues until time value 66 when the brakes are released. The driverselected ratio range remains constant, as shown at 68, from thebeginning to the end of section B where the engine start is triggered.At a later time an accelerator pedal is advanced to create an increasingaccelerator position plot 70.

Preconditions that exist before initiation of the vehicle hold featureinclude a stopped engine, a closed driver door, a released park brake, areleased accelerator pedal, stand-still of the vehicle, application ofthe brakes, and a position of the ratio selector indicates that thevehicle is in Drive.

FIG. 1b demonstrates the change in brake line pressure and mastercylinder brake pressure. Before the driver brake release is detected, apressure hold at 70 is higher than the driver brake pressure input levelshown at 72 in FIG. 1b . The level 72 is sufficient to prevent vehicleroll back. If the brake line pressure drops to a level that isinsufficient for vehicle hold purposes as shown at 74, an active brakepressure buildup trajectory using a pressure source (electric auxiliarypump 24) as shown at 76, until a value shown at 78 is reached. When theengine start is complete, the wheel torque hold feature is inactive asshown at 80.

FIG. 1c shows the WTDS hold feature status after the driver brakerelease is detected and the brake pressure hold begins. The brake isreleased at 86. The hold feature is inactive, as shown at 90, after theengine start-up at or after the WTDS timer runs out as shown at 88. Thehold feature will not be enabled if no engine stop has been experiencedor the transmission is not in gear during an engine start-up.

FIG. 1d shows a WTDS timer status. The timer set time is zero initially,as shown at 92. The timer may be set to a value as shown at 94. Afterengine start-up is detected, the brake pressure drops immediatelyirrespective of the WTDS timer status. This is shown at 95. The timercan then be reset after being inactive. When the engine start istriggered, the timer runs down to the level shown at 96. Stepped plot 98illustrates the timer run-down.

FIG. 1e is a time plot of changes in vehicle speed. When the enginestart is triggered at time B in FIG. 1e , the vehicle speed increasesbecause of vehicle creep as shown at 102. This increase may continueuntil a vehicle launch is made at 104.

FIG. 1f shows the trigger points where engine start is requested. Anengine start/stop scheduler changes from an engine stop state 106 to anengine start state 108.

FIG. 1g shows an engine speed plot at the beginning of a launch. Idlespeed in FIG. 1f is shown at 110. Engine cranking begins at the end oftime A. Combustion torque transients during the cranking mode are shownat 112. The transients continue until the end of time B. A peak enginespeed is reached at 114. The engine speed will eventually decrease, asshown at 116, until the idle speed is reached at 118 when the creep modeis in effect. Following the creep mode, a launch occurs as shown at 120.

The brake release flag is set and the braking function is set to aninactive state when a certain powertrain state condition is satisfied.The engine speed signal is used as a powertrain state indicator. Thebrake release is activated when an engine started flag is set, which isdetermined by either of the following two conditions. First, a certainengine speed threshold has been passed and the engine start-up speedpeak is detected at 114. Second, a certain high engine speed thresholdhas been passed for a certain calibratable time duration. The enginespeed signal is both low pass filtered and high pass filtered to getrelatively smooth engine speed signals and the derivative of thesignals, which indicates a rate of change of the engine speed.

FIG. 1h shows a crankshaft torque at the wheels brake torque and roadload at the wheel throughout the vehicle hold engine start-up event.Brake torque is shown at 122, wheel torque is shown at 124, and enginetorque at the wheels is shown at 126. Brake torque after an engine startis shown at 128. The plot at 124 is equal to the algebraic sum of theplot at 128 and the plot at 126.

The plot in FIG. 1h assumes that the transmission is engaged in gearduring the engine start-up process.

The engine is running between time B and the end of time C. Before timeB, the engine is off. There may be about a 300-500 millisecond delay indelivering wheel creep torque. The delay depends on how fast the enginecan be started up. This is illustrated by the dotted line oval at 130 inFIG. 1h . The crank torque at the wheels peaks, as shown at 132, whentime B is reached. This corresponds approximately to a peak negativebrake torque as shown at 134.

FIG. 1i shows the time values for the various engine start-up flags. Thestart-up flag during engine starting occurs at 136. An engine start flagis shown at 138 and an engine running flag is shown at 140.

In FIG. 1b , it is seen that an active pressure build-up will occur whenthe driver releases the brake pedal too quickly. When that occurs, aspreviously described, there is an active pressure build-up supplied byan auxiliary pump, which supplies more pressure to maintain a sufficientpressure to keep the vehicle from rolling back.

FIG. 2 is a flowchart that demonstrates the steps followed during theengine start-up events that occur as shown in FIGS. 1a through 1i . Atthe start of the routine of FIG. 2 entry conditions are established at142. If no engine stop state occurs at 146 due to a master brakecylinder pressure being less than the initial combustion torquemitigation pressure (hold pressure) as shown at step 148, the routinewill proceed to the step 150 where it is determined whether automaticengine stop and vehicle standstill are in place. If they are not, theroutine will not continue. If the inquiry at 150 is positive, a decisionis made at step 152 as to whether the transmission is in gear. If it isnot, the braking function will be inhibited as shown at 144. If theinquiry at 152 is positive an inquiry is made at step 154 regardingwhether the brake hold system is available to support the vehicle holdfunction. If the inquiry is positive, the vehicle hold function isenabled at 156. If it is not available, the routine returns to thebeginning

The inquiry at 158 determines whether the driver brake release isdetected. If the brake remains applied by the driver, the routine willnot continue. If the driver releases the brake, the vehicle holdfunction will be activated as shown at 160. An inquiry is made as towhether the brake pressure is sufficient to hold the vehicle as shown at162. The active brake pressure build function then begins at step 164.This is graphically illustrated in FIG. 1b . Step 162 also isillustrated in FIG. 1 b.

The step at 164 is an optional step. It is possible for the routine tocontinue from step 162 directly to the engine automated restart commandstep at 166.

The step previously discussed at 158 is illustrated graphically in FIG.1 c.

The step at 166 determines whether an engine automated restart has beencommanded. If it has not been commanded the routine then can return tostep 168. It is determined at 168 whether the driver has reapplied thebrakes. If he has not, the routine can then continue as previouslydescribed. If the driver has reapplied the brakes, the vehicle holdfeature will be deactivated, as shown at 170. If the commanded masterbrake cylinder is zero as detected at 172 the routine then can continueas previously described at step 154.

If the inquiry at step 166 is positive, the timer is set at step 168.This step is illustrated in FIG. 1d as previously described.

If the timer counts down as determined at 170, the vehicle hold functionreset conditions are tested at 172. It is determined at this stepwhether the accelerator pedal is applied by the driver or whether thebrake pedal reapply has been initiated by the driver. If neither ofthese conditions is present, the routine will proceed directly to step176 where it is determined if the timer runs down or if an engine speedpeak is detected, or if the commanded master brake cylinder pressure iszero. If the inquiries at 176 are negative, an automatic engine restartis attempted at 169 if the engine has stalled.

If the inquiries at 172 are positive, the routine can proceed directlyto steps 174 and 178. Upon a brake release determined at 178, theroutine can return to the beginning

A detection of an engine speed peak as indicated at 180 will result inan immediate termination of the previously described brake holdconditions, as shown at 176.

An adaptive load estimation and wheel brake pressure and torquethreshold determination are made at 181. This is illustrated by line 182in FIG. 1b . The brake pressure threshold is used at step 164 to signala build-up of brake hold pressure as shown at 76 in FIG. 1b . Theconditions that will terminate the brake hold feature are: the timer isreset, or an engine speed peak is detected during cranking, or themaster brake cylinder pressure 182 in FIG. 1b falls to zero as the brakeis released.

OVERVIEW

The disclosed embodiment of the invention includes a method forpreventing micro-hybrid vehicle motion before engine startup using anautomatic engine stop-start function during a driving event. The methoduses a driver operated service brake to build up and/or maintain braketorque during an engine off state when the vehicle is stopped.

The disclosed embodiment of the invention applies a brake torquehold/build function at a torque level higher than the actual driver'sbrake torque input.

The disclosed embodiment of the invention uses the brake torquehold/build function after detecting driver brake release.

The disclosed embodiment enables the brake torque hold/build functionafter detecting an engine stop in drive when the vehicle is instandstill and the estimated road gradient is in a low gradient range(−3%˜7%).

The disclosed embodiment uses engine speed to make brake releasedecisions after the engine is commanded to start in Drive.

The brake hold is further coordinated with the engine startup processsuch that the function extends to a case where possible engine startupfails.

Although an embodiment of the invention has been disclosed, it will beapparent to persons skilled in the art that modifications may be madewithout departing from the scope of the invention. All suchmodifications and improvements therein are covered by the followingclaims.

What is claimed is:
 1. A method for controlling a vehicle having an engine, wheel brakes, and a controller configured to perform the method, comprising: stopping the engine with the vehicle stopped on a surface with a gradient; maintaining the wheel brakes applied during an engine automatic restart; and lowering fluid pressure applied to the wheel brakes to release the wheel brakes during the engine automatic restart in response to detecting an engine speed peak and a predetermined engine speed.
 2. The method of claim 1 wherein maintaining the vehicle wheel brakes applied comprises increasing the fluid pressure applied to the wheel brakes for a predetermined brake pedal position after a brake pedal is released.
 3. The method of claim 1 wherein the vehicle includes a starter motor and wherein: stopping the engine comprises stopping the engine in response to application of the wheel brakes; and maintaining the wheel brakes applied comprises maintaining the wheel brakes applied while the starter motor is cranking the engine during the engine automatic restart when the gradient is within a predetermined range.
 4. The method of claim 1 wherein lowering the fluid pressure applied to the wheel brakes to release the wheel brakes further comprises releasing the wheel brakes in response to at least one of depressing a brake pedal, depressing an accelerator pedal, and expiration of a calibrated time following brake application during the engine automatic restart. 